Numerous mice have been hyperimmunized with elutriator-purified human monocytes. After screening the fusion hybrid cell lines with 3 different assays, it appears that the antibody product of 2 lines recognizes antigens found only on monocytes, 3 lines recognize antigens found on monocytes and lymphocytes, and 2 lines recognize only lymphocytes. These monoclonal antibodies and the antigens they recognize will be further characterized. Monocytic leukemias will be screened for these antigens. The ability of the antibodies to modulate monocyte function will be assayed. Subsets of monocytes will be sought.